This invention relates to a clock signal conditioning circuit, and particularly, although not exclusively, to a clock signal conditioning circuit for use in a "smart card".
The term "smart card" is commonly used to refer to a portable, credit-card size card containing an integrated circuit having a central processing unit (CPU), input and output terminals and memory. Such cards have no on-card power supply and rely on the application of external power and clock signals in order to actively function. The clock signal applied externally to such a card must be a periodic signal with a frequency greater than a minimum frequency f.sub.min and less than a maximum frequency f.sub.max.
Under some conditions the duty cycle of the externally applied clock signal may vary, and parasitic effects may cause random transients (glitches) in the externally applied clock signal. The result of such duty cycle variation and/or glitches in the externally applied clock signal may be that the application program being run by the CPU does not execute properly, causing uncontrolled mis-functioning of the smart card.